Boron nitride carbon alloy solar cells

ABSTRACT

Solar cells fabricated from p-n junctions of boron nitride nanotubes alloyed with carbon are described. Band gaps of boron nitride carbon alloys are tailored by controlling carbon content in the boron nitride nanotubes. High efficiency solar cells can be fabricated by tailoring the band gap of boron nitride carbon alloy nanotubes, and using these nanotubes for fabricating solar cells u. Because boron nitride carbon alloy nanotubes are transparent to most wavelengths of light, the wavelengths not converted to electrons (i.e., absorbed) at a first p-n junction in a solar cell will pass through the stack to another p-n junction in the stack having a different band gap. At each successive p-n junction, each of which has a different band gap from the other p-n junctions in the stack, more wavelengths of light will be converted into electricity. This dramatically increases the efficiency of solar cells.

This application is continuation of U.S. Non-Provisional applicationSer. No. 15/970,114, filed on May 3, 2018, which is a continuation ofPCT Appl. No. PCT/US16/60971, filed on Nov. 8, 2016, which claimsbenefit of U.S. Provisional Appl. No. 62/252,781, all of which areincorporated by reference herein in their entireties.

BACKGROUND

The present disclosure relates generally to high efficiency andmechanically durable solar cells. Specifically, the present disclosurerelates to solar cells fabricated from boron nitride, and, inparticular, boron nitride carbon alloys.

Most commercially available solar cells convert about 12% of incidentsunlight energy into electricity. This relatively low conversion rate ofincident sunlight to electricity (often referred to as “efficiency”) isa significant factor in the cost of renewable energy generated fromsolar cells.

One cause of the low efficiency of conventional solar cells is therelatively narrow band gap used in the semiconductor devices ofconventional solar cells. That is, semiconductor materials generallyused to fabricate commercial solar cells have band gaps from between 1.1eV to 1.7 eV. For “single junction” solar cells, in which constituentp-n junctions use a single type of p material and a single type of nmaterial for all p-n junctions in the solar cell, the band gap isnarrow—on the order of several tenths of an electron volt (eV) centeredaround the band gap at the interface between the materials used to makethe junction. This narrow range of energy in the band gap results in anarrow range of wavelengths present in the spectrum of sunlight that canbe converted to electricity by the solar cell.

Further complicating this is that the spectrum of sunlight available forconversion at the Earth's surface is not continuous over a spectrum ofwavelengths. Sunlight transmitted in the vacuum of space can beapproximated as black body radiation, which includes a continuous rangeof wavelengths of light. Sunlight received at the Earth's surface,however, has interacted with components of the Earth's atmosphere. As aresult, many wavelengths of light originally present in sunlight (i.e.,when in the vacuum of space) are absorbed by components of theatmosphere before reaching the Earth's surface. This complicates theefficient conversion of sunlight into electricity using conventionalsolar cells because some solar cells may be configured to convertwavelengths of light to electricity that have been absorbed by theEarth's atmosphere prior to reaching the solar cell.

Multi junction solar cells that include a variety of band gaps in theconstituent p-n junctions are configured to convert a wider range ofwavelengths to electricity. However, the interfaces of p-n junctions ofmulti junction solar cells must be epitaxial, which substantiallyincreases the cost of solar cell fabrication.

SUMMARY

Example 1 of the present disclosure includes a solar cell that includesa first layer of boron nitride carbon alloy having an n-type dopant; anda second layer of boron nitride carbon alloy having a p-type dopant, thesecond layer in contact with the first layer to form a p-n junction,wherein a first band gap of the first layer and a second band gap of thesecond layer are each determined by a first carbon content of the alloyof the first layer and a second carbon content of the alloy of thesecond layer, respectively. Example 2 of the present disclosure includesthe subject matter of Example 1, further comprising a third layer ofboron nitride carbon alloy having an n-type dopant, the third layer incontact with the second layer, wherein a third band gap of the thirdlayer is different from the first band gap of the first layer anddifferent from the second band gap of the second layer, the third bandgap of the third layer determined by a third carbon content of the alloyof the third layer. Example 3 includes the subject matter of any ofclaim 1 or 2, wherein the boron nitride carbon alloy comprises boronnitride carbon alloy nanotubes. Example 4 of the present disclosureincludes the subject matter of Example 2, wherein an interface betweenthe second layer and the third layer converts a first range of lightwavelengths into electricity, the first range of wavelengths transmittedthrough the first layer and the second layer. Example 5 of the presentdisclosure includes the subject matter of any of Examples 1-4, whereinan efficiency of the solar cell is greater than 50%. Example 6 of thepresent disclosure includes the subject matter of any of Examples 1-5,wherein the first layer and the second layer are transmit greater than50%, 60%, 70%, 80%, 90% or 95% of light in the range of from 400 to 700nm. Example 7 of the present disclosure includes the subject matter ofany of Examples 1-6, wherein the boron nitride carbon alloys of thefirst layer and the second layer comprises boron atoms and nitrogenatoms arranged in an array of hexagonal unit cells; and carbon atomssubstituted for at least one of a first portion of boron atoms and asecond portion of nitrogen atoms. Example 8 of the present disclosureincludes the subject matter of Example 7, wherein lattice parameters ofa hexagonal unit cell changes less than 5% upon substitution of a carbonatom for either one of a boron atom or a nitrogen atom. Example 9 of thepresent disclosure includes the subject matter of Example 7, whereincarbon atoms are disposed at a lattice site formerly occupied by one ofa boron atom and a nitrogen atom. Example 10 of the present disclosureincludes the subject matter of Example 7, wherein carbon atoms are notdisposed at interstitial sites of a boron nitride nanotube crystal.Example 11 of the present disclosure includes the subject matter of anyof Examples 1-10, wherein the solar cell has a density of less than 1.0g/cc. Example 12 of the present disclosure includes the subject matterof any of Examples 1-11, wherein the solar cell can convert threeseparate wavelengths of light to electricity, the three separatewavelengths of light spaced from one another by at least 100 nm. Example13 of the present disclosure includes the subject matter of any ofExamples 1-12, wherein the solar cell is a non-planar solar cell.Example 14 of the present disclosure includes the subject matter ofExamples 1-13, wherein the non-planar solar cell has at least onearcuate portion. Example 15 of the present disclosure includes thesubject matter of any of Examples 1-14, wherein one of the boron nitridecarbon alloy layers comprises at least twice the carbon content ofanother boron nitride carbon alloy layer of the solar cell. Example 16of the present disclosure includes the subject matter of any of Examples1-14, in which the solar cell is greater than 12%, greater than 15% orgreater than 20% efficient at conversion of sunlight to electricity.Example 17 of the present disclosure includes the subject matter of anyof Examples 1-16, wherein a solar cell of any of the preceding claimscomprising a housing, a front sheet and a back sheet.

Example 18 includes a method of converting sunlight to electricity, themethod including exposing a first boron nitride carbon alloy layer and asecond boron nitride carbon alloy layer to sunlight, the first layercomprising a first carbon concentration and the second layer comprisinga second carbon concentration that may be the same or different from thefirst carbon concentration, one of the first layer and the second layerbeing p-doped and the other being n-doped; exposing a third boronnitride carbon alloy and fourth boron nitride carbon alloy layer tosunlight that has passed through the first layer and second layer, thethird layer and the fourth layer each including carbon at aconcentration different than that of the first and second layer; andproducing an electric current at the junction of the first and secondlayer and at the junction of the third and fourth layer. Example 19includes the subject matter of Example 18, wherein the layers arecomprised of boron nitride carbon alloy nanotubes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic illustration of a structure of boron nitride inplan view, in an embodiment.

FIG. 1B is a schematic illustration of a structure of a boron nitridecarbon alloy in plan view, in an embodiment.

FIG. 1C is a schematic illustration of a structure of a boron nitridecarbon alloy nanotube, in an embodiment.

FIG. 2 is a graph of illumination power versus sunlight wavelength asmeasured at the Earth's surface, in an embodiment.

FIG. 3 is a graph of band gap (E_(g)) versus composition for a boronnitride carbon alloy, in an embodiment.

FIG. 4 is a schematic illustration of a solar cell of the presentdisclosure including multiple p-n junctions, each of which is designedto convert a different wavelength of sunlight to electricity, in anembodiment.

The figures depict various embodiments of the present disclosure forpurposes of illustration only. One skilled in the art will readilyrecognize from the following discussion that alternative embodiments ofthe structures and methods illustrated herein may be employed withoutdeparting from the principles described herein.

DETAILED DESCRIPTION Overview

Embodiments of the present disclosure include solar cells fabricatedfrom p-n junctions of boron nitride (“BN”) nanotubes that have beenalloyed with carbon. Band gaps of boron nitride carbon (“BNC”) alloysare tailored by controlling carbon content in the BN nanotubes. Bytailoring the band gap of BNC alloy nanotubes and fabricating solarcells using appropriately tailored BNC alloys, high efficiency solarcells are fabricated. The high efficiency of BNC alloy solar cells hasat least two sources. First, BNC alloy nanotubes have, in someembodiments, a band gap (E_(g)) tailored to match wavelengths of lightmost prevalent at the surface of the Earth. Second, in some embodiments,layers of BNC nanotube alloys are used to fabricate a solar cell stack.Each BNC nanotube alloy layer can have a different band gap configuredto convert a particular wavelength (or range of wavelengths) intoelectricity. Because BNC alloy nanotubes are substantially transparentto most wavelengths of light, the wavelengths not converted to electrons(i.e., absorbed) at a first p-n junction will pass through the stack toanother p-n junction in the stack having a different E_(g). At eachsuccessive p-n junction, each of which can have a different band gapfrom the other p-n junctions in the stack, more wavelengths of lightwill be converted into electricity. This dramatically increases theefficiency of BNC nanotube alloy solar cells.

Boron Nitride and Boron Nitride Carbon Alloy Structure

FIG. 1A illustrates the structure and composition of BN. BN and BNCalloys can occur in both nanotube and planar sheet allotropes. Whileembodiments of the present disclosure are generally directed to BNnanotubes alloyed with carbon, the structure in FIG. 1A (and FIG. 1B)are presented in a planar sheet configuration for convenience ofexplanation.

As shown, BN (whether occurring in a nanotube configuration or a planarsheet) is generally structured as adjacent hexagons. A single hexagonwithin a sheet is referred to herein as a “unit cell” when convenient.At each of the vertices of each hexagon is disposed one of either aboron atom or a nitrogen atom. Generally the boron and nitrogen aredisposed at alternating vertices. The boron and nitrogen atoms arebonded together by sp² covalent bonds, with a slight ionic character tothe bond caused by the relative electronegativities of the boron andnitrogen atoms.

A plurality of these hexagons, such as the ones shown in FIG. 1A, arebonded together to form a BN sheet or nanotube. BN sheets are, in someexamples, formed macroscopically as one or more of (1) a monolayer (2)abi-layer, (3) a tri-layer. Structurally, BN sheets are comparable tothe carbon allotrope graphene because of the hexagonal symmetry within asheet and the formation of the sheets into monolayers, bilayers,trilayers, and combinations thereof. The inter-sheet spacing of bilayersor trilayers of BN is smaller than that of graphene. This is due to thepartial ionic character of the sp² bonds between the boron and nitrogenatoms, which provides more inter-sheet attraction than is presentbetween carbon graphene sheets. BN nanotubes are generally formed by asingle layer of atoms configured into a tubular allotrope but caninclude more layers extending over some or all of a tube. For example,tubes may be singled walled, double walled, triple walled ormultiwalled.

FIG. 1B illustrates an example of a BNC alloy, in an embodiment. As inFIG. 1A, FIG. 1B is shown in a planar configuration for convenience ofexplanation even though some embodiments of the present disclosuredescribed below are presented in the context of BNC alloy nanotubes usedto fabricate solar cells. In FIG. 1B, some of the boron atoms and someof the nitrogen atoms in the BN structure have been substituted bycarbon atoms. Because of the similarity in atomic size of boron (atomicnumber 10), carbon (atomic number 12), and nitrogen (atomic number 14),carbon atoms can substitute for either of boron atoms or nitrogen atomswithin a single unit cell or within an array of unit cells of BN. Thissubstitution forms a BN-carbon alloy in either one of a sheet or ananotube and does not change the symmetry of the unit cells and does notchange the dimensions of a unit cell (i.e., lattice parameters) by morethan 5%.

Alloying BN with carbon is unlike doping intrinsic semiconductors with adopant. In a semiconductor doping process, a semiconductor host isimplanted with a dopant that is either a donor of electrons or a donorof holes. The donor, often implanted using a high energy ion beam, isinserted into the crystal structure of the host at an interstitial siteor a crystallographic defect, such as a vacancy. Donor concentrations ofdopants are in a range of parts per million with respect to the hostcomposition. An example is implantation of arsenic (a hole donor) intosilicon. The atomic size of arsenic is substantially smaller thansilicon and therefore can conveniently fit into interstitial sites inthe crystal structure of silicon.

Alloying BN with carbon is unlike this doping process in that carbonatoms substitute directly on lattice sites in the molecular structurefor either boron atoms, nitrogen atoms, or both, without disturbing theBN unit cell. That is, carbon alloying atoms reside at a lattice siteand not an interstitial site or defect location. In the boron nitridealloy, carbon atoms take the place of boron or nitrogen atoms in thelattice structure. Thus, carbon alloying atoms are present in the BNcrystal structure in stoichiometric amounts.

While the symmetry of BN is essentially unchanged upon alloying withcarbon, the electronic structure does change. This will be explained inmore detail in the context of FIG. 2

FIG. 1C schematically shows a BNC alloy nanotube of the presentdisclosure. The hexagonal unit cell symmetry and composition of BN andBNC, as described above, is unchanged in nanotubes.

Boron Nitride Carbon Alloy Synthesis and Properties

Alloying of BN nanotubes with carbon is accomplished in any one ofseveral possible fabrication methods. In one example method, the alloyednanotubes can be made by forming the BNC alloyed nanotubes in situ, bysubstituting boron and carbon into a carbon nanotube (CNT), or bysubstituting carbon into a BN nanotube. In one example method, a plasmaassisted chemical vapor deposition (“CVD”) is used to react methylchloride gas with nitrogen to produce BN nanotubes. In another examplemethod, borazine gas is reacted in a plasma of a plasma CVD process toproduce BN nanotubes. The BN nanotubes, regardless of the method used toproduce them, are then alloyed with carbon by heating the nanotubes toapproximately 800° C. in the presence of flowing ethylene gas or othercarbon containing gas. In another example method, BN nanotubes arealloyed with carbon by dissolving Tetracyanoquinodimethane (TCNQ) inacetone. BN nanotubes are then dipped into the TCNQ acetone solution,with carbon substituting for boron and nitrogen atoms in the BN nanotubehexagonal structure, as described above. TCNQ doped BNC alloy nanotubesare n-doped. In different embodiments, a carbon alloyed BN nanotube cancomprise greater than 1%, greater than 3%, greater than 5% or greaterthan 10% carbon, on a molar basis. Alloyed BNC nanotubes may be void ofany other elements or may be essentially void of any other elements. Forexample, BNC nanotubes may consist of or consist essentially of carbon,boron and nitrogen.

BN monolayers and nanotubes have an E_(g), prior to doping with carbon,of about 5.6 eV. Materials having a value of E_(g) this high aretypically considered insulators. However, upon alloying BN monolayersand/or nanotubes with carbon, the E_(g) of BN monolayers and nanotubeschanges as a function of carbon content. This enables BNC alloys to besynthesized into solar cells tailored to convert a range of wavelengthsof sunlight into electricity.

FIG. 3 is a graph showing band gap (E_(g)) versus composition for aboron nitride carbon alloy, in an embodiment. In embodiments, dependingon the amount of carbon used to alloy BN nanotubes, E_(g) of theresulting BNC alloy ranges from 0.7 eV to 2 eV. In some examples, a bandgap of a BNC alloy can be adjusted by application of a transverseelectric field. This graph can be found in Journal of Physics, volume76, number 6, published June 2011.

Once fabricated, a BNC alloy is doped to p-type by simple exposure tooxygen. A BNC alloy, whether undoped or p-doped, can be converted to ann-type material by exposure to borazine, monovalent gold, TCNQ, andcombinations thereof.

Boron Nitride Carbon Alloy Solar Cells

One feature of embodiments of the present disclosure, described above,is the high efficiency of solar cells comprising BN nanotubes. There areat least two reasons for this: (1) BNC alloys are mostly transparent(i.e., transmit greater than 50%, 60%, 70%, 80%, 90% or 95% of incidentintensity) to many wavelengths of light (barring occasional scatteringfrom crystal defects) including those from 400 nm to 700 nm and (2) theband gap of BNC nanotube alloys can be tailored over a very wide rangeof eV values. An example solar cell of the present disclosure thatutilizes these two advantages is illustrated in FIG. 4.

As shown in FIG. 4, an example solar cell 400 of the present disclosureincludes a plurality of stacked layers, 404, 408, 412, and 416, each ofwhich is fabricated from BNC alloy nanotubes that have been tuned withdifferent carbon contents to have different band gaps. While four layersare shown in the example solar cell 400, other embodiments are notlimited to four layers and may have more than four or fewer than fourlayers in a stack.

The layers 404, 408, 412, and 416 in the example solar cell 400 arealternately doped n-type and p-type. When configured in a stack, thealternating n-type and p-type layers form p-n junctions at eachinterface. Furthermore, as described above, the band gaps of the layersat each interface are configured to convert a particular range ofwavelengths of light to electricity, as described above. Because thelayers 404, 408, 412, and 416 are substantially transparent to mostwavelengths of light, the solar cell 400 has an extremely highefficiency because any wavelength of light not converted to electricityat one interface travels deeper into the stack, for possible conversionto electricity at a different interface exhibiting a different band gap.

For convenience of explanation, light incident onto the solar cell 400includes three wavelengths of light, λ₁, λ₂, λ₃. The layer 404 is dopedp-type and the layer 408 is doped n-type. The BNC nanotube alloys ineach of 404 and 408 are alloyed with a carbon content so that thecorresponding band gap converts light of wavelength λ₁ to electricity.As with most BNC nanotube alloys, layers 404 and 408 are transparent tomany wavelengths of light, permitting the transmission of light havingwavelengths λ₂, λ₃ deeper into the stack of layers of the solar cell400. As shown, the transmitted wavelength λ₂ travels to the interfacebetween n-type layer 408 and p-type layer 412. Analogous to the p-njunction between layers 404 and 408, the p-n junction formed at theinterface between layers 408 and 412 has a band gap designed (via carboncontent) to absorb light having a wavelength λ₂. The light having awavelength λ₃ is transmitted through the layers 408 and 412 to a p-njunction formed at the interface between layers 412 and 416. As shown,the layer 412 is doped p-type and the layer 416 is doped n-type. Theband gap of the p-n junction formed by the interface of the layers 412and 416 is configured (via carbon content) to convert light ofwavelength λ₃ to electricity. In some embodiments, higher energywavelengths are absorbed at deeper levels in the stack. In additionalembodiments, some layers having the same or similar band gap may berepeated in a stack to capture light that was not absorbed at a higherlevel in the stack. In other embodiments, the bottom surface of thestack may include a mirror to reflect any transmitted light back upwardsthrough the stack of BNC nanotube layers.

Consistent with the above example, the number of layers (andcorresponding p-n junctions formed at the interfaces of the layers) andthe associated band gaps are configurable to convert any desired rangeof light wavelengths to electricity. In some examples, the band gaps ofthe various p-n junctions are configured to convert some or all of thewavelengths of light available at the Earth's surface. In otherexamples, the p-n junctions are configured to maximize electricityproduction of black body radiation for space applications.

In some embodiments, the various layers of an example solar cell (suchas the example solar cell 400) are fabricated from BNC alloy nanotube“felts” in which the constituent nanotubes are not oriented within alayer. In other examples, the layers are fabricated from BNC alloynanotube fabrics in which the nanotubes are oriented eitherperpendicular to a horizontal plane of the layers (i.e., parallel to az-axis) or parallel to the horizontal plane of the layers (i.e.,parallel to an x-axis). In still other embodiments, the layers arefabricated from woven threads and/or fabrics of BNC alloys which arealloyed and doped according to embodiments of the present disclosure andplaced into intimate contact to form p-n junctions. In some cases, thesolar cell can be flexible. For example, the layers may be curvedrepeatedly to a radius of less than 1 meter, less than 10 cm or lessthan 1 cm. Single BNC alloy nanotubes can also be used in someembodiments to form p-n junctions, as described above.

Benefits and Advantages

Benefits of solar cells fabricated using BNC alloys according to thepresent disclosure include (1) a lighter weight than conventionalsilicon or III-V semiconductor solar cells owing to a BNC density ofapproximately 0.8 g/cc compared to densities of silicon and III-Vsemiconductors of approximately 2 to 4 g/cc. BNC alloys are also moreflexible that conventional semiconductors, with BNC alloys able to bebent into a curved or arcuate conformation to a radius of curvature ofseveral nanometers whereas conventional semiconductors are inflexibleand brittle. This enables solar cells to be fabricated that conform toan underlying surface (even a non-planar underlying surface), have atleast one arcuate portion, or are even cylindrical. BNC alloys are alsothermally stable (with little or no change in physical or electricalproperties) up to about 900° C. in air. This benefit is particularlyadvantageous in space applications (e.g., powering satellites) or otherapplications in which the service environment is demanding (e.g.,military applications).

Another advantage of BNC alloys is their transparency to most of thevisible, near IR, and near UV wavelengths of light (e.g., fromapproximately 100 nm to approximately 1000 nm). This transparencyenables the layered configuration, and resulting high efficiency,described above.

BNC alloys are resistant to radiation, with little or no change incrystallographic structure, p-n junction structure, and mechanicalproperties even when exposed to radiation of space that is unfiltered bythe Earth's atmosphere. This benefit is advantageous for spaceapplications and is in distinct contrast to multi junction solar cellsfabricated from conventional semiconductor materials that do degradewhen exposed to space radiation.

BNC allows are mechanically stronger than conventional solar cellmaterials and can withstand the forces involved in a space launch.

In other examples, embodiments of the present disclosure are applied toproduce a high fidelity display.

FURTHER CONSIDERATIONS

The foregoing description of the embodiments of the disclosure has beenpresented for the purpose of illustration; it is not intended to beexhaustive or to limit the claims to the precise forms disclosed.Persons skilled in the relevant art can appreciate that manymodifications and variations are possible in light of the abovedisclosure.

The language used in the specification has been principally selected forreadability and instructional purposes, and it may not have beenselected to delineate or circumscribe the inventive subject matter. Itis therefore intended that the scope of the disclosure be limited not bythis detailed description, but rather by any claims that issue on anapplication based hereon. Accordingly, the disclosure of the embodimentsis intended to be illustrative, but not limiting, of the scope of theinvention, which is set forth in the following claims.

1-20. (canceled)
 21. A solar cell having at least three layers, thesolar cell comprising: a first layer of boron nitride carbon alloynanotubes having a first carbon concentration; a second layer of boronnitride carbon alloy nanotubes having a second carbon concentrationgreater than the first; and a third layer of boron nitride carbon alloynanotubes having a third carbon concentration greater than the second,wherein each of the layers exhibits a band gap different than that ofthe other layers.
 22. The solar cell of claim 21 wherein each of the atleast three layers has a density of less than 2 g/cc.
 23. The solar cellof claim 21 wherein each layer comprises nanotube felts.
 24. The solarcell of claim 21 wherein each layer comprises nanotubes orientedperpendicular to a plane of the layers.
 25. The solar cell of claim 21wherein each layer comprises nanotubes oriented horizontally to a planeof the layers.
 26. The solar cell of claim 21 wherein one of the layershas a first wavelength of light for which it is most efficient atconverting to electricity and at least one of the remaining layers issubstantially transparent to that first wavelength.
 27. The solar cellof claim 21 wherein each of the at least three layers transmits morethan 50% of incident light.
 28. The solar cell of claim 21 wherein eachof the at least three layers exhibits a band gap of 0.7 eV to 2 eV. 29.The solar cell of claim 21 wherein the carbon atoms reside at a latticesite and not at an interstitial site or defect.